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    <meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1, minimum-scale=1, user-scalable=no">
    <meta name="description" content="Use Viewer to start building new applications or easily embed Cesium into existing applications.">
    <meta name="cesium-sandcastle-labels" content="Beginner, Showcases">
    <title>Cesium Demo</title>
    <script type="text/javascript" src="../Sandcastle-header.js"></script>
    <script type="text/javascript" src="../../../Build/CesiumUnminified/Cesium.js" nomodule></script>
    <script type="module" src="../load-cesium-es6.js"></script>
</head>
<body class="sandcastle-loading" data-sandcastle-bucket="bucket-requirejs.html">
<style>
    @import url(../templates/bucket.css);
</style>
<div id="cesiumContainer" class="fullSize"></div>
<div id="loadingOverlay"><h1>Loading...</h1></div>
<div id="toolbar"></div>
<script id="cesium_sandcastle_script">
function startup(Cesium) {
    'use strict';
//Sandcastle_Begin
// This example illustrates a Callback Property, a property whose
// value is lazily evaluated by a callback function.
// Use a CallbackProperty when your data can't be pre-computed
// or needs to be derived from other properties at runtime.
var viewer = new Cesium.Viewer('cesiumContainer');
viewer.clock.shouldAnimate = true;

var startLatitude = 35;
var startLongitude = -120;
var endLongitude;
var startTime = Cesium.JulianDate.now();

// Add a polyline to the scene. Positions are dynamic.
var isConstant = false;
var redLine =  viewer.entities.add({
    polyline : {
        // This callback updates positions each frame.
        positions : new Cesium.CallbackProperty(function(time, result) {
            endLongitude = startLongitude + 0.001 * Cesium.JulianDate.secondsDifference(time, startTime);
            return Cesium.Cartesian3.fromDegreesArray([startLongitude, startLatitude, endLongitude, startLatitude], Cesium.Ellipsoid.WGS84, result);
        }, isConstant),
        width : 5,
        material : Cesium.Color.RED
    }
});

var startCartographic = Cesium.Cartographic.fromDegrees(startLongitude, startLatitude);

// use scratch object to avoid new allocations per frame.
var endCartographic = new Cesium.Cartographic();
var scratch = new Cesium.Cartographic();
var geodesic = new Cesium.EllipsoidGeodesic();

// Calculate the length of the line
function getLength(time, result) {
    // Get the end position from the polyLine's callback.
    var endPoint = redLine.polyline.positions.getValue(time, result)[1];
    endCartographic = Cesium.Cartographic.fromCartesian(endPoint);

    geodesic.setEndPoints(startCartographic, endCartographic);
    var lengthInMeters = Math.round(geodesic.surfaceDistance);
    return (lengthInMeters / 1000).toFixed(1) + ' km';
}

function getMidpoint(time, result) {
    // Get the end position from the polyLine's callback.
    var endPoint = redLine.polyline.positions.getValue(time, result)[1];
    endCartographic = Cesium.Cartographic.fromCartesian(endPoint);

    geodesic.setEndPoints(startCartographic, endCartographic);
    var midpointCartographic = geodesic.interpolateUsingFraction(0.5, scratch);
    return Cesium.Cartesian3.fromRadians(midpointCartographic.longitude, midpointCartographic.latitude);
}

// Label the polyline with calculated length.
var label = viewer.entities.add({
    position : new Cesium.CallbackProperty(getMidpoint, isConstant),
    label : {
        // This callback updates the length to print each frame.
        text: new Cesium.CallbackProperty(getLength, isConstant),
        font : '20px sans-serif',
        pixelOffset : new Cesium.Cartesian2(0.0, 20)
    }
});

// Keep the view centered.
viewer.trackedEntity = label;
//Sandcastle_End
    Sandcastle.finishedLoading();
}
if (typeof Cesium !== 'undefined') {
    window.startupCalled = true;
    startup(Cesium);
}
</script>
</body>
</html>
